Fraxinus
pennsylvanica has opposite, compound leaves with short-stalked leaflets
that are not strongly whitened beneath. The upper edge of the leaf scar
is not strongly concave as in F. americana, and the first pair
of lateral buds are generally not separated from the terminal bud as in
F. nigra. The above not withstanding, F.pennsylvanica is
extremely variable and caution is recommended in identification, because
it is frequently misidentified as either F. nigra or F. americana.
The twigs range from glabrous to densely pubescent (the primary character
used to split this species into varieties when Red and Green Ash are distinguished),
the shape of leaves and leaf scars is variable, and it is not uncommon
for some lateral buds to be sub-opposite rather than clearly opposite.
The youngest leaves of seedlings may be simple rather than compound. The
flower buds are highly prone to deformation, sometimes confusing observers.
See paragraph 2 of the discussion of F. americana
for a comparison of characters among the Ash species.

Fraxinus pennsylvanica is by far the commonest
ash species in southern Wisconsin. It is present in many of the disturbed
wet forests and is also a very aggressive invader of old fields and
other disturbed, open sites. See the discussion of Black
Ash.